1. Field of the Invention
The present invention relates to a semiconductor photo-detecting element and, more particularly, to an avalanche photodiode (APD).
2. Description of the Related Art
The planar type superlattice avalanche photodiode (hereinafter abbreviated as APD) shown in FIG. 7 has been reported as a high-speed, high-sensitivity photo-detecting element for next-generation optical communication systems (Patent Document 1 and Non-patent Document 1).
Such conventional photo-detecting elements have high gain bandwidth product (GB product) and low-noise owing to the effect of an InAlAs/InAlGaAs superlattice multiplication layer on an increase in the ratio of the ionization rate and have a structure that only a stable InP planar pn junction is exposed to the surface. Therefore, they are characterized by being high-reliability elements.
More specifically, the layered structure of a conventional photo-detecting element consists, on a semiconductor substrate, of a buffer layer of the first conductivity type, a light-absorbing layer of the first conductivity type, a field buffer layer of the first conductivity type, a multiplication layer, an etching stopper layer, a buffer layer of the second conductivity type, and a contact layer of the second conductivity type.
For this layered structure, an impurity of the first conductivity type is diffused from the substrate surface to a peripheral portion of an isolation trench formed in the periphery of the element, and the upper surface of the peripheral portion is formed as an electrode of the first conductivity type, whereas an electrode of the second conductivity type is formed on the surface of the center of the element.
The width of the isolation trench is large relative to the distance over which the impurity is diffused, and no short circuit due to the diffusion of the impurity occurs.
The electrodes of the first and second conductivity type s are separated from each other by the isolation trench which is formed by performing etching from the surface to the etching stopper layer (after the formation of the isolation trench, a dielectric film is formed), and function as two electrodes of a diode.
Also, by doping an impurity to the bottom of this isolation trench, the electric field distribution is adjusted so that good breakdown characteristics can be obtained.
Because photo-detecting elements fabricated in this manner permit light-electricity conversion at high speeds and with high sensitivities, they are used especially effectively in receivers for optical communication and in devices for optical measurement.
Patent Document 1: Japanese Patent Laid-Open No. 7-312442
Non-patent Document 1: Watanabe et al., IEEE, Photonics Technol. Lett., pp. 827-829, vol. 8, 1996
In the structure of the conventional technology, as shown in FIG. 8, electric fields applied to the multiplication layer and the etching stopper layer have almost the same strength.
The APD of the planar type structure shown in the related art uses a superlattice structure as the multiplication layer, and this structure is characterized by multiplication from a low electric field. In the case of a superlattice structure, an electric field applied to the multiplication layer is low and, therefore, the breakdown electrical field strength of the etching stopper layer is not exceeded. For this reason, the dark current problem is not a great problem.
On the other hand, when a multiplication layer having a higher GB product is to be obtained, there has been known a method which involves increasing the value of GB product, for example, by reducing the film thickness of a single layer of InAlAs as the multiplication layer without using a superlattice structure.
InP is often used as a material for the etching stopper layer. The reason is that the etching selective ratio can be set at a high value compared to materials, such as InAlAs, InGaAs and InAlGaAs. On the other hand, InP has a low breakdown electrical field strength compared to InAlAs and InAlGaAs. For this reason, InP is a material desirable for an etching stopper. However, in a case where the GB characteristics are to be improved by reducing the film thickness of the electronic multiplication layer (increasing the field strength applied to the electronic multiplication layer), a high field strength exceeding the breakdown electrical field strength of the etching stopper layer is applied to the etching stopper layer and a very large dark current is generated in the etching stopper layer, deteriorating the multiplication characteristics and increasing noise of the device.
Incidentally, in addition to InP, as the etching stopper layer InxGa(1-x)AsyP(1-y) (0≦x≦1.0, 0≦y≦1.0) and AlAsSb can also be used under lattice matching conditions with InP.
Therefore, it is impossible to apply the technique for fabricating a high-sensitivity semiconductor photo-detecting element by using a multiplication layer in which the GB product is Improved by reducing the film thickness of the multiplication layer.